docs: def parse(self, limit=None):
          """
          Override Source.parse()
          Args:
              :param limit (int, optional) limit the number of rows processed
          Returns:
              :return None
          """
          if limit is not None:
              LOG.info("Only parsing first %d rows", limit)
  
          rgd_file = '/'.join(
              (self.rawdir, self.files['rat_gene2mammalian_phenotype']['file']))
          # ontobio gafparser implemented here
          p = GafParser()
          assocs = p.parse(open(rgd_file, "r"))
  
          for i, assoc in enumerate(assocs):
              if 'relation' in assoc.keys():
                  self.make_association(assoc)
              if limit is not None and i > limit:
                  break
          return